The present invention relates to a method for the preparation of a sintered porous body of a metal or ceramic.
Porous bodies of a metal are known in the prior art and can be prepared by several methods including (1) blowing of a gas into a melt of the metal, (2) mixing hollow balloons to a melt and (3) admixing of a melt with a blowing agent such as titanium hydride TiH2 followed by blowing of the agent.
These prior art methods, however, have their respective problems and disadvantages. For example, these methods have a limitation in the selection of the metal which must have a relatively low melting point enabling easy handling of the melt or in the selection of the blowing agent which must have a gas-evolution temperature in good matching with the melting behavior of the melt. The rheological behavior of the metal melt such as viscosity is also an important factor in order to minimize escape of the blowing gas from the body of the melt. Due to these situations, it is the present status of the art that the prior art methods have been applied only to the preparation of porous bodies of aluminum and a few others.
Proposals have been made for an improvement of the prior art methods in which a composition prepared by blending a metal powder and a blowing agent is subjected to molding by powder forging or by extrusion into a shaped body which is subjected to foaming and sintering by heating at a temperature in the vicinity of the gas-release temperature of the blowing agent. This method is also hardly applicable to a variety of metals other than aluminum and the like due to the requirement for matching between the gas-release temperature of the blowing agent and the melting behavior of the metal.
As a further improvement of the prior art methods, proposals are made according to which a compound or a slurry comprising a metal powder and a binder resin is admixed with a burnt-off material which is lost by heating for sintering of the metal powder or the compound or slurry is subjected to direct foaming. These methods are also not free from problems. Namely, the former of these methods has a limitation in the highest porosity of the porous bodies obtained by the method which can hardly exceed 80%.
As a variation of the latter of these methods for direct foaming, by which a porosity of as high as 95% or higher can be accomplished, there is known a method in which a foaming precursor blend is prepared by admixing a metal powder to a foaming premix for the preparation of a polyurethane foam and the precursor blend is subjected to a foaming treatment in the same manner as in the preparation of a foamed polyurethane body. Even though the highest porosity obtained by this method can be so high as mentioned above with sufficiently good fineness of the foam cells, the foamed body has an open cell texture rather than a closed cell texture desirable in most applications in addition to the problem due to the residual silica or phosphorus-containing impurities derived from the foaming conditioner agent in the polyurethane formulation.
As a modification of the prior art methods, a method is proposed in Japanese Patent Kokai No. 9-87704 in which a slurried blend with admixture of a blowing agent is sheeted by means of a doctor blade under heating to effect simultaneous foaming and drying of the sheet into a foamed precursor body having a porosity of 95% or even higher. A problem in this method is a limitation in the maximum thickness of the foamed body, which can hardly exceed 1 cm, because the foaming treatment is effected for a sheet of the slurried blend of which the thickness is under limitations by the weight of the slurried blend per se and the binding performance, e.g., viscosity, of the binder resin rarely enabling to obtain a bulky foamed body which should desirably have a closed cell texture.
As is understood from the above description of the present status of the art, it is eagerly desired to develop an industrial method for the preparation of a sintered porous body of a metal powder in a bulky form having a porosity of 95% or higher with a closed cell texture.